1. Field of the Invention
The present invention relates to enhancement-mode Group III-N HEMTs and, more particularly, to a manufacturable enhancement-mode Group III-N HEMT with a reverse polarization cap.
2. Description of the Related Art
Group III-N high electron mobility transistors (HEMTs) have shown potential superiority for power electronics due to their wider bandgap and high electron saturation velocity. These material properties translate into high breakdown voltage, low on-resistance, and fast switching. Group III-N HEMTs can also operate at higher temperatures than silicon-based transistors. These properties make group III-N HEMTs well suited for high-efficiency power regulation applications, such as lighting and vehicular control.
FIG. 1 shows a cross-sectional view that illustrates a conventional enhancement-mode group III-N HEMT 100. As shown in FIG. 1, enhancement-mode group III-N HEMT 100 includes a substrate 110, and a layered structure 112 that touches the top surface of substrate 110. Substrate 110 is commonly implemented with SiC because SiC has a reasonably low lattice mismatch (˜3%) and a high thermal conductivity. SiC substrates, however, are expensive and limited in size. Substrate 110 is also commonly implemented with Si due to the low cost of Si and access to Si processing infrastructure.
Layered structure 112, in turn, includes a buffer layer 114 that touches the top surface of substrate 110, a channel layer 116 that touches the top surface of buffer layer 114, and a barrier layer 118 that touches the top surface of channel layer 116. Barrier layer 118, in turn, has a recess 119 that has a bottom surface that lies vertically above the bottom surface of barrier layer 118.
Buffer layer 114, channel layer 116, and barrier layer 118 are each typically implemented with one or more sequential group-III nitride layers, with the group-III including one or more of In, Ga, and Al. For example, barrier layer 118 is commonly formed from AlGaN, while channel layer 116 is commonly formed from GaN. Thus, channel layer 116 is implemented with a group III-N material that is different from the group III-N material that is used to implement barrier layer 118.
In addition, layered structure 112 is conventionally formed by growing layered structure 112 on substrate 110 using epitaxial deposition techniques such as metal organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE). After layered structure 112 has been formed, recess 119 is formed with a conventional mask and etch step.
Buffer layer 114 provides a transition layer between substrate 110 and channel layer 116 in order to address the difference in lattice constant and to provide a dislocation-minimized growing surface. However, when substrate 110 is formed from silicon, it is difficult to grow buffer layer 114 thicker than 2-3 um on a 6-inch substrate due to the stress, the subsequent bowing of the wafer, and cracking of the nitride films.
As further shown in FIG. 1, enhancement-mode group III-N HEMT 100 also includes a gate dielectric 120 that touches the top surface of layered structure 112 and lines recess 119, and a metal gate 122 that touches the top surface of gate dielectric 120. In addition, enhancement-mode group III-N HEMT 100 includes a metal source 124 and a metal drain 126 that make ohmic contacts through barrier layer 118. Metal source 124 and metal drain 126 are horizontally-spaced apart and electrically connected to channel layer 116.
In operation, as discussed in Mishra et al., “AlGaN/GaN HEMTs—An Overview of Device Operation and Applications”, Proceedings of the IEEE, Vol. 90, No. 6, June 2002, pp. 1022-1031, the channel layer and the barrier layer of a HEMT have different polarization properties and band gaps that induce, as shown in FIG. 1, the formation of a two-dimensional electron gas (2DEG) 130 that lies at the top of the channel layer. The 2DEG 130, which has a high concentration of electrons, is similar to the channel in a conventional field effect transistor (FET).
Further, in an enhancement-mode device, the 2DEG 130 is normally depleted of electrons under the gate, and thereby normally an off device. Thus, when ground is placed on metal gate 122, no current flows from metal drain 126 to metal source 124 by way of the 2DEG 130. However, when ground is placed on metal source 124, a positive voltage is placed on metal drain 126, and a positive voltage greater than the threshold voltage is placed on metal gate 122, a current flows from metal drain 126 to metal source 124 by way of the 2DEG 130.
Thus, during the operation of enhancement-mode group III-N HEMT 100, each time group III-N HEMT 100 is turned on and off, a large voltage is placed on and then removed between metal gate 122 and metal drain 126. The application and removal of a large voltage over many thousands of times stresses parts of the HEMT, such as gate dielectric 120, and leads to the eventual failure of enhancement-mode group III-N HEMT 100. As a result, it is desirable to have a dielectric-free enhancement-mode group III-N HEMT.
In “Gate Injection Transistor (GIT)—A Normally-Off AlGaN/GaN Power Transistor Using Conductivity Modulation,” IEEE Transactions on Electron Devices (TED), Vol. 54, Issue 12, 2007, pp. 3393-3399, Uemoto et al proposed an enhancement-mode group III-N HEMT that utilized a p-doped (Mg) cap of AlGaN in lieu of gate dielectric 120. The p-type dopant depletes the 2DEG, thereby forming a normally off device. The device is then turned on by, for example, placing ground on metal source 124, a positive voltage on metal drain 126, and a positive voltage that is greater than the threshold voltage on metal gate 122.
In the enhancement-mode group III-N HEMT using the hole injection principle, the turn off time is unfortunately slowed down due to the need to extract the holes. Another issue is that it is difficult to remove the p-doped AlGaN layer, which lies on the AlGaN barrier layer, from regions where it is not needed to form the cap of AlGaN, thereby increasing variability. If a GaN cap is used instead of the cap of AlGaN (which was used in lieu of gate dielectric 120), such as from UC Santa Barbara or Ferdinand-Braun-Institut, the above issues are simplified, but the maximum allowable voltage on the gate terminal is limited by junction turn on, which may not meet the requirement of circuit designers.
In “AlGaN/GaN HEMTs with Thin InGaN Cap Layer for Normally Off Operation,” IEEE Electron Device Letters, Vol. 28, Issue 7, 2007, pp. 549-551, Mizutani et al proposed an enhancement-mode group III-N HEMT that utilized a reverse polarization cap of InGaN in lieu of gate dielectric 120. The reverse polarization of InGaN raises up the conduction band and depletes the 2DEG, thereby forming a normally off device. The device is then turned on by, for example, placing ground on metal source 124, a positive voltage on metal drain 126, and a positive voltage that is greater than the threshold voltage on metal gate 122.
One of the weaknesses of the enhancement-mode group III-N HEMT proposed by Mizutani is that the device can not be reliably manufactured on a large scale because it is difficult to form a patterned InGaN structure that sits on an AlGaN barrier layer. Mizutani proposed fabricating the enhancement-mode group III-N HEMT by first growing a thin InGaN cap layer on the top surface of the barrier layer. Then, after a metal gate has been formed (metal deposition, mask, and etch), the metal gate itself is used as a mask to remove the exposed regions of the thin InGaN cap layer.
However, it is difficult to remove the exposed regions of the thin InGaN cap layer without removing portions of the underlying AlGaN barrier layer due to a lack of a selective etch to do so. Thus, there is a need for an enhancement-mode group III-N HEMT that reduces the problems associated with a gate dielectric and that allows for manufacturable fabrication. In addition, there is also a need to increase the maximum allowable gate voltage in order to better meet the needs of circuit designers.